1. Field of the Invention
This invention relates to pitches which are excellent as starting materials for the production of carbon fibers.
2. Description of the Prior Art
At present, carbon fibers are produced mainly from polyacrylonitrile as the starting material. However, polyacrylonitrile as the starting material for carbon fibers is disadvantageous in that it is expensive, tends not to retain its fibrous shape when heated for stabilization and carbonization and is carbonized in a low yield.
In view of the above, there have recently been reported a number of methods for producing carbon fibers from pitch. In cases where pitch is used as the starting material for producing carbon fibers, it is expected to obtain carbon fibers at a low cost since pitch is inexpensive and may be carbonized in a high carbonization yield of as high as 85-95%. However, carbon fibers obtained from pitch raise a problem that although they have higher tensile modulus, their tensile strength is poorer than those obtained from polyacrylonitrile. If, thus, there is found a method for solving the problem and further improving the pitch-derived carbon fibers in tensile modulus, such a method will render it possible to produce carbon fibers having high tensile strength and high tensile modulus at a low cost from pitch.
There was recently reported a method for producing carbon fibers having improved tensile modulus and tensile strength, which comprises heat treating a commercially available petroleum pitch to obtain a pitch containing optically anisotropic liquid crystals called "mesophase", providing the pitch containing the mesophase as a precursor pitch (such a pitch in the melt spinning step being hereinafter referred to as "precursor pitch"), melt spinning the precursor pitch, infusibilizing (making infusible) the melt spun pitch and then carbonizing or further graphitizing the pitch so infusibilized (Japanese Laid-open Patent Application No. 49-19127).
However, whether or not pitch may form liquid crystal therein depends on various factors. In addition, the resulting liquid crystals will greatly depend for their structure, softening point, viscosity and other physical properties on the type of starting pitch. The above-indicated Japanese Laid-open Patent Application deals with a method for producing a pitch containing the mesophase (hereinafter referred to as "mesophase pitch"), but it does not refer to anything about a starting pitch for producing a mesophase pitch of good quality therefrom. As mentioned before, it depends greatly on a starting pitch whether or not a mesophase pitch of good quality may be obtained therefrom. If a very desirable starting pitch is obtained, then it will be possible to produce therefrom carbon fibers having excellent tensile modulus and tensile strength. Therefore, it is an important problem in the field of this art how to find such a very desirable starting pitch.
For example, coal tar pitch contains carbon blacklike, quinoline-insoluble and infusible substances, and these undesirable substances cause the non-uniformity of the precursor pitch thereby not only degrading the spinnability but also having adverse effects on the tensile strength and tensile modulus of the resulting carbon fibers.
In contrast, many of commercially available petroleum pitches and synthetic pitches scarcely contain any quinoline-insoluble and infusible substances, however, they will produce quinoline-insoluble and high molecular weight substances when heat treated to prepare a precursor pitch therefrom. More particularly, when these pitches are heat treated, they will cause both thermal decomposition and polycondensation whereby the low molecular weight ingredients gradually convert to quinoline-insoluble high molecular weight ones. Further, the high molecular weight ingredients so formed will, in turn, form further high molecular weight ones, accompanied with a raise in softening point of the pitches. If these quinoline-insoluble ingredients are similar to the carbon black-like substances in coal tar, they will have adverse effects in the spinning and its subsequent steps as mentioned. In addition, even if the quinoline-insoluble ingredients are those which are different from the carbon black-like substances, the existence of the quinoline-insoluble substances in large amounts and the raise in softening point in the pitches will have adverse effects in the melt spinning step. More particularly, for melt spinning the precursor pitches, it is necessary to raise a spinning temperature to such an extent that the pitches have a viscosity sufficient to be melt spun. Thus, if the precursor pitches have too high a softening point, then the spinning temperature must naturally be raised with the result that the quinoline-insoluble ingredients form further high molecular weight ones, and the pitches cause their pyrolysis with light fraction gases being evolved thereby rendering it practically impossible to obtain homogeneous pitches and carry out melt spinning of the pitches.
As is seen from the above, it is essential that precursor pitches have a comparatively low softening point and a viscosity suitable to enable them to be spun. Furthermore, the precursor pitches must not be such that they contain a substantial amount of volatile ingredients at the time of spinning and carbonization.
For this reason, the quinoline-insoluble ingredients are removed by filtration under pressure, separation by solvent, or other suitable means to prepare precursor pitches for producing carbon fibers (Japanese Laid-Open Patent Application Nos. 47-9804, 50-142820, 55-1342 and 55-5954). However, the methods disclosed in these applications are not desirable from the economical point of view since they require complicated equipment and incur an increased cost.
It is the most preferable if there may be used, as the starting pitch, an excellent pitch which will not produce quinoline-insoluble high molecular weight ingredients when heated for for preparing the mesophase pitch.
The present inventors made intensive studies in an attempt to obtain such an excellent pitch. As a result, we obtained an excellent starting pitch which will inhibit the production of high molecular weight ingredients, have an optimum viscosity, and be able to have a composition allowing the aromatic planes to be easily arranged in order in the step of preparing precursor pitches.